Glass-to-Metal Seals
The distinct lack of thermodynamic affinity between glasses and metal alloys is the primary reason that joining of these materials has historically involved considerable technical complexity. The basic requirements for a strong, hermetic glass-to-metal seal are chemical bonding (i.e. electronic structure via atomic contact) and minimal stress differentials at the glass/metal interfaces, along with favorable stress gradients in the interfacial zones. Although methods have been established for modeling residual stress in glass seals and it is well known that stress differentials at glass-to-metal seal interfaces are minimized by matching the thermal expansion coefficients of the components, the dependence of stress gradients in the interfacial zones on composition gradients that form during the fusing of the seals is not as well recognized.